1. Field of the Invention
The invention relates to inductive couplers for non-contacting power transmission, particularly for computer tomographs. With these, electric power mainly needed for operating an X-ray tube is transmitted from a stationary side to a rotating side of a gantry of a computer tomograph. Here the transmission is effected without contact by means of an inductive rotary joint that is built up similarly to a transformer in which the primary side and the stationary side are rotatable with respect to each other.
2. Description of Relevant Art
With units that are movable relative to each other, such as radar installations or also computer tomographs, similarly as in the case of linearly movable units such as crane installations or conveyor vehicles, it is frequently necessary to transmit electrical energy between movable units. In order to transmit this energy without contact, inductive couplers are preferably used. These have the advantage over mechanical slide tracks or also slip rings that abrasion, wear, mechanical effort for moving the coupler, and also the maintenance effort is substantially less. The term “inductive coupler” here refers to a circuit for generating an alternating-current voltage and an inductive transmission device or rotary joint for energy transmission between two parts that are rotatable relative to each other, and in particular to rotatable parts.
As disclosed in U.S. Pat. No. 7,197,113, for example, inductive rotating transmission devices have magnetic cores of iron or ferrite material and at least one winding on each side of units that are rotatable relative to each other. An alternating current is fed into a first winding and tapped off via a second winding that is movable relative thereto.
U.S. Pat. No. 7,054,111 shows a complete circuit of an inductive power transmission system for computer tomographs, including the associated power electronics.
With conductively coupled slip rings it is simple to transmit a predetermined voltage from a stator side to a rotor side. Here only the relatively small ohmic losses must be taken into account. With inductive rotary transmission devices, the stray inductance of a rotary transmission device plays a substantial part. It represents a frequency-dependent impedance that substantially affects the transmission properties of the rotating transmission device. This stray inductance depends upon various factors such as the inductance of the windings of a stator side and a rotor side, and also upon the magnetic structure. Now, in order to transmit electrical energy through a rotating transmission device of this kind, a series capacitance is connected in series for compensation. With this, a series resonance circuit results. This has an impedance of zero at its resonance frequency and allows for a transmission of large power. For control of the power flow, the operating frequency can be chosen to differ from the resonance frequency.
Instead of a series resonance circuit, a parallel resonance circuit also can be built-up by connecting a capacitance in parallel. The properties described in the following apply similarly to a parallel resonance circuit. At its resonance frequency the resonance circuit has an impedance of almost zero and here allows for a transmission of large power. The output voltage can be controlled by changing the impedance, which is performed by changing the switching frequency.
The inductances in a resonance circuit represent frequency-dependent impedances that substantially affect the transmission properties of the rotating transmission device. These inductances depend upon various factors such as the structure and the permeability of the magnetic circuit, the structure of the winding and, in particular, the airgap between the stator side and the rotor side. These factors are not constant in series fabrication, but are subject to particular tolerances. Various measures are known for maintaining the output voltage on the rotating side within acceptable limits for all values, occurring in the series, of the significantly involved component parts concerned—in particular the resonance capacitor, the matching transformer, and the inductive rotating transmission device. One possibility is to measure the output voltage and to feed back this output value to the stationary side. For this, however, a rotating transmission device is needed, which causes additional cost and requires space.
Another possibility is to build-in additionally a converter stage, mostly a DC-DC converter, on the rotating side between the secondary side of the rotating transformer and the output. In these converter stages frequently buck or boost converters are used, but other converters such as Zeta or Cuk converters are also possible. The input voltage of this downstream connected converter stage may fluctuate within a wide range, with the output voltage being kept constant. This solution, however, needs an additional converter on the rotating side, which increases cost and also weight and volume of the arrangement.